With all those over-40 women having babies, does this mean you can wait indefinitely if you hope to get pregnant? Not really. A woman’s peak of fertility is about 25 years of age. After that, “it’s all downhill.” The likelihood of becoming pregnant drops dramatically well before you reach menopause, which is what many women think of as the end of their fertile years. A great number of those after-40 pregnancies are the results of medical interventions such as in vitro fertilization and donor eggs from 25 year olds. Unlike our male counterparts who keep producing new sperm every 74 days, women are given their complement of eggs way before they are even born and there are no more new eggs to be produced. Therefore, at 36 years of age, a woman’s eggs are 36+ years old with all the attendant risks that accompany any aging process. According to the March of Dimes:

“At age 25, a woman has about a 1-in-1,250 chance of having a baby with Down syndrome; at age 40, the risk increases to 1-in-100 chance; and at 45, the risk of carrying a child with a chromosomal anomaly such as Down syndrome, continues to rise to 1-in-30 chance.”

The advent of artificial reproductive technologies virtually transforms a woman’s “biological clock” into a perpetual calendar, but not without risks. In studies, babies born via in vitro fertilization have been shown to have a higher risk of birth defects.

If an older woman doesn’t mind having a baby who carries none of her DNA, she may opt for a donor egg from a younger woman, which is then fertilized by her husband and the embryo transferred into her uterus. Many of the older celebrities have chosen this route for their family planning.

Medical interventions, while they seem miraculous when they work, aren’t guaranteed to be successful. Just as in getting pregnant the old-fashioned way, your chances of success drop the older you are. In vitro fertilization will result in a live birth among women past 40 only 6 to 10 percent of the time versus a 30 to 35 percent success rate among women younger than 35.

Nature’s message is clear, and unfortunately, it doesn’t offer any leeway in difficult economic times or while you are working your way up the corporate ladder: if you want to start a family, you’re more likely to be successful if you begin well before you turn 40.